London 2012 organisers have confirmed that "a few thousand" volunteers will have to be trained up and deployed to help fill a gap in the number of security personnel required at the Olympics next summer.

Following a review process over the summer it emerged that about 21,000 security staff would be required inside Olympic venues and training camps, more than double the original estimates.

Paul Deighton, chief executive of the organising committee, said a large proportion of the extra recruits would have to be trained by G4S, which is providing at least 10,000 guards of its own and overseeing the security operation.

The difference is expected to be made up of military personnel (including reservists and Territorial Army recruits), volunteers and newly trained security guards undergoing a programme called Bridging the Gap.

"We would expect to have to train a significant number of them. Even the military, who have been trained for all sorts of things, won't necessarily have been trained to run a vehicle-screening programme, which is where they might be deployed," Deighton said.

The volunteers chosen to be deployed for security tasks are likely to be utilised at the beginning of the screening process to speed the flow of people through the gates.

"It's essentially a welcome activity. The whole chain of activity of taking people through the screening is part of the security numbers. One of the things we did at the test events was look at different techniques for getting people through the screening as quickly as possible."

Deighton said the discussions with the Home Office over the huge leap in security numbers had proceeded amicably.

"We've been working on this with the Home Office for a long time. We have a highly collaborative relationship. The discussions they've been having with the military have been very constructive. People understand that we're doing the biggest thing we've ever done in this country, which has a significant security component," he said.

"I think the military having a role is sensible and from my point of view I think it's terrific – great people, readily trained. It makes perfect sense as long as we remember they have their operational requirements to deal with as well. That's for the Home Office, the Ministry of Defence and government to work out – we're just the recipient of whatever they're able to provide us."

The final numbers of security staff, and the likely significant increase to the £282m provided from the public funding package to cover security at the venues, is likely to be confirmed next week. The increased costs, likely to run to hundreds of millions of pounds, will be drawn from the £2bn contingency fund built into the Games' original £9.3bn budget.

Separately, £600m has been allocated to the police for security outside the venues. The Home Office said last year that it should be able to deliver for £475m.